Method and system for determining gas turbine tip clearance

ABSTRACT

A system for sensing at least one physical characteristic associated with an engine including a turbine having a plurality of blades turning inside a casing, the system including: a pressure sensor coupled substantially adjacent to the casing and including at least one output; a port in the turbine casing for communicating a pressure indicative of a clearance between the blades and casing to the pressure sensor; a cooling cavity substantially surrounding the pressure sensor; and, an inlet for receiving fluid from the engine and feeding the fluid to the cooling cavity to cool the pressure sensor; wherein, the pressure sensor output is indicative of the clearance between the blades and casing.

RELATED APPLICATION

This application claims the benefit of priority of U.S. ProvisionalPatent Application Ser. No. 60/582,289, filed Jun. 23, 2004, entitled“Gas Turbine Tip Clearance Measurement Using Case-Mounted PressureTransducers”, the entire disclosure of which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to gas turbine engines and theiroperation, and more particularly to gas turbine engine tip clearancemeasurement systems.

BACKGROUND OF THE INVENTION

It is well known that tip clearance leakage is one of the primary lossmechanisms in axial flow compressors and turbines of a gas turbineengine. Tip clearance loss translates into lost efficiency, higher fuelcosts and thus higher operating costs. More particularly, over theoperating life of an engine such as an aircraft engine, tip clearanceincreases over time, due at least in part to mechanical rubs betweenrotating blades and stationary casing and erosion. This clearancedeterioration is a leading driver for engine performance deterioration,which often manifests in increased fuel burn and exhaust gastemperatures (EGT). The FAA mandates that an engine be removed formaintenance/overhaul once the EGT reaches an upper limit.

It is desirable therefore to maintain tip clearance as low as possiblein an effort to minimize related losses throughout the engine-operatingenvelope. One way of achieving this is to use Active tip ClearanceControl (ACC) systems, such that clearance levels are adjusted forengine operating conditions, and throughout the operating cycle. For anyACC concept to work effectively, real-time tip clearance data isrequired as part of the control algorithm. However, current tipclearance sensors are believed to be deficient in certain regards.

Accordingly, an alternative tip clearance measurement technique andsystem for accomplishing tip clearance measurement is highly desirable.

SUMMARY OF THE INVENTION

A system for sensing at least one physical characteristic associatedwith an engine including a turbine having a plurality of blades turninginside a casing, the system including: a pressure sensor coupledsubstantially adjacent to the casing and including at least one output;a port in the turbine casing for communicating a pressure indicative ofa clearance between the blades and casing to the pressure sensor; acooling cavity substantially surrounding the pressure sensor; and, aninlet for receiving a fluid such as compressed air from the engine andfeeding the compressed air to the cooling cavity to cool the pressuresensor; wherein, the pressure sensor output is indicative of theclearance between the blades and casing.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding of the present invention will be facilitated byconsideration of the following detailed description of the preferredembodiments of the present invention taken in conjunction with theaccompanying drawings, in which like numerals refer to like parts, and:

FIG. 1 illustrates gas turbine tip clearance sensor system according toan aspect of the present invention; and,

FIG. 2 illustrates gas turbine tip clearance sensor system according toan aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding, while eliminating, for the purpose ofclarity, many other elements found in typical gas turbine engines andmethods of making and using the same, and pressure sensing systems andmethods of making and using the same. Those of ordinary skill in the artmay recognize that other elements and/or steps may be desirable inimplementing the present invention. However, because such elements andsteps are well known in the art, and because they do not facilitate abetter understanding of the present invention, a discussion of suchelements and steps is not provided herein.

FIG. 1 illustrates a schematic cross section of an exemplary turbinesystem 100 including a pressure transducer 110 mounted substantiallyadjacent to, including for example, within the interior of, turbine case120. Transducer 110 measures pressure on the turbine case 120, andprovides a signal indicative of the sensed pressure via leads (notshown) electrically coupled thereto.

Turbine system 100 may include an engine assembly that takes the form ofa conventional gas turbine engine. In operation, blades 140 of engine100 rotate past port 150 which communicates the pressure at the turbinecasing 120 to transducer 110. As a result of blade rotation, thepressure sensed by transducer 110 varies. As a blade passes and obscuresport 150, the inlet of port 150 is essentially closed and the pressurecommunicated to transducer 110 is essentially the ambient staticpressure. The inlet to port 150 becomes un-obscured after the bladepasses. At this point, the communicated and sensed pressure rises to amaximum pressure indicative of blade 140 loading. This cyclic processrepeats as each of the turbine blades 140 passes port 150.

As is understood, tip clearance size affects the blade loading. This isdue to leakage flows from one side of the blade to the other across theclearance gap. Hence, the unsteady pressure field exerted upon port 150is a function of tip clearance size. The functional dependence betweenthe two tip clearance and the pressure signature as measured by thetransducer may be established through computer modeling and/orcalibration testing, for example. Thus, one may derive real-time tipclearance data from sensing the unsteady pressure signature resultingfrom turbine blades passing by a case mounted pressure transducer.

As will be understood by those possessing an ordinary skill in thepertinent arts, pressure transducer 110 may have a frequency responsecapability roughly 5-10 times that of the blade passing frequency inorder to resolve the flow structure at the blade tip region. Forexample, the blade passing frequency for a high-pressure turbine in atypical modem gas turbine engine may be around ten kilohertz (10 KHz).Accordingly, transducer 110 may have a frequency response on the orderof about 50 KHz-100 KHz. Such high frequency operation may requiretransducer 110 to be mounted close to turbine casing 120—as a physicallyextending port 150 may serve to essentially low-pass filter the pressuresignature resulting from turbine blades 140 passing port 150.

The output of pressure transducer 110 may optionally be provided to asignal processing and conditioning electronics module 130 remotelylocated within the system 100. Sensor 110 and /or signal processor 130may provide one or more signals indicative of an operating condition ofthe engine assembly 100, such as turbine tip clearance.

Signal processing and conditioning electronics module 130 may include aprocessor and memory, by way of example only. “Processor”, as usedherein, refers generally to a computing device including a CentralProcessing Unit (CPU), such as a microprocessor. A CPU generallyincludes an arithmetic logic unit (ALU), which performs arithmetic andlogical operations, and a control unit, which extracts instructions(e.g., code) from memory and decodes and executes them, calling on theALU when necessary “Memory”, as used herein, refers to one or moredevices capable of storing data, such as in the form of chips, tapes ordisks. Memory may take the form of one or more random-access memory(RAM), read-only memory (ROM), programmable read-only memory (PROM),erasable programmable read-only memory (EPROM), or electrically erasableprogrammable read-only memory (EEPROM) chips, by way of furthernon-limiting example only. The memory utilized by the processor may beinternal or external to an integrated unit including the processor. Forexample, in the case of a microprocessor, the memory may be internal orexternal to the microprocessor itself. Of course, module 130 may takeother forms as well, such as an electronic interface or ApplicationSpecific Integrated Circuit (ASIC).

As is well understood by those possessing an ordinary skill in thepertinent arts, in general an axial flow turbine engine includes acompressor, combustion area and turbine. In compressor applications, thecasing temperature is at or below 1300 degrees Fahrenheit (1300° F.). Inthe turbine section, the metal temperature can reach as high as 2500° F.According to an aspect of the present invention, transducer cooling mayused. According to another aspect of the present invention, pressuretransducers for turbine clearance measurement may be air cooled,optionally using the same cooling air that may be used to cool theturbine casing.

Referring still to FIG. 1, there is shown a cooling chamber 160substantially surrounding transducer 110 and having a cooling air inlet170 and outlet 180. Cooling air for inlet 170 may be drawn from acompressor of turbine system 100, and thus have a temperature around1300° F., for example. The cooling air may circulate through chamber160, cooling transducer 110 and/or the immediate environment it issubjected to, and then exit outlet 180. A high temperature pressuretransducer, such as model WCT-250 or WCT-312 cooled by air of waterpressure sensor, commercially available from Kulite SemiconductorProducts, Inc. the assignee hereof, may be used in combination with sucha cooling scheme to provide a system that can reliably operate in a hightemperature, high pressure turbine environment.

Referring now to FIG. 2, there is shown a turbine system 200 accordingto an aspect of the present invention. Like references have been used inFIGS. 1 and 2 to designate like elements of the invention. Hence, adetailed discussion of those common elements will not be repeated. Insystem 200, cooling air is again fed into a chamber 160 substantiallysurrounding pressure transducer 110 via inlet 170. The cooling air isagain discharged using an outlet 180. However, outlet 180 of system 200discharges spent cooling air into the main gas path, i.e., into theturbine. Outlet 180 may discharge into port 150, such that pressuresensing port 150 of system 200 will have a net air outflow, forming adischarge jet into the turbine. This allows an interaction between thedischarge jet and the passing turbine blades 140. This interaction mayenhance the sensing of the unsteady pressure as a function of tipclearance size. Parameters, such as discharge jet velocity and flow rateof the cooling chamber, inlet and outlet may be chosen to maximize thesensitivity of the sensed unsteady pressure signal as a function of tipclearance.

By way of further, non-limiting example only, the cooling airflow inFIG. 2 is modulated by the relative motion of turbine blades orairfoils. The cooling air is first modulated by the interaction betweenthe cooling air and unsteady pressure field around each turbine blade.The unsteady pressure fluctuations will modulate cooling airflow rate,therefore affecting air pressure measured by the pressure sensor. Thecooling air is also modulated by the interaction with the turbine bladesthemselves. When the turbine blades periodically pass over the coolingair discharge jet, a blockage effect occurs when the turbine blade isaligned with the discharge jet, whereas no or little blockage is presentwithout such an alignment. This on and off blockage effect modulates thecooling airflow rate, again impacting unsteady pressure measurements.The amount of blockage, and the resultant pressure fluctuations, willdepend on blade geometry and tip clearance size. As blade geometry isknown, tip clearance may be deduced.

According to an aspect of the present invention, by sizing the coolingair discharge and cooling chamber geometries, one may “acousticallytune” the effect on transducer 110 so as to maximize pressurefluctuations due to tip clearance changes, thus increasing tip clearancemeasurement accuracy.

According to an aspect of the present invention, transducer 110 may alsobe utilized to measure turbine rotational speed. Transducer 110 sensesthe turbine blade passing frequency, by sensing the unsteady pressurefield generated each time a turbine blade 140 passes port 150. Usingthis frequency, together with the known configuration of the turbineitself, such as the number of blades installed on the turbine wheel, onemay readily deduce turbine shaft speed. Such a shaft speed sensor mayprove more reliable, and physically lighter than conventional magneticspeed transducers. Further, as a same transducer may be used to providemultiple functionality according to an aspect of the present invention,additional cost savings to the engine system as a whole may be realized.

According to an aspect of the present invention, tip clearance may beadjusted using a conventional methodology responsively to the output ofthe pressure transducer.

Those of ordinary skill in the art may recognize that many modificationsand variations of the present invention may be implemented withoutdeparting from the spirit or scope of the invention.

1. A system for sensing at least one physical characteristic associatedwith an engine comprising a turbine having a plurality of blades turninginside a casing, said system comprising: a casing coupled with theturbine, said casing having a longitudinal port, said port extendingtransverse to said casing with a first end of said port opening insidesaid casing; and a pressure transducer mounted substantially adjacent tosaid casing, in said port, said pressure transducer facing said firstend, said port communicating to said pressure transducer a firstpressure in the presence of said turbine blades and a second pressurewhen said turbine blade is not present; wherein, said pressuretransducer has a frequency response capability which is a multiple ofthe blade passing frequency, and a processor receiving said first andsecond pressure outputs to determine a clearance between said blades andcasing.
 2. The system of claim 1, further comprising: a cooling cavitysubstantially surrounding said pressure transducer; and an inlet forreceiving a fluid from said engine and feeding said fluid to saidcooling cavity to cool said pressure transducer.
 3. The system of claim2, wherein said engine further comprises a compressor, and said inlet issubstantially adjacent to said compressor.
 4. The system of claim 3,wherein said fluid is compressed air.
 5. The system of claim 4, furthercomprising an outlet for receiving said compressed air from said cavityand expelling said compressed air.
 6. The system of claim 5, whereinsaid outlet is external to said casing.
 7. The system of claim 2,wherein said cavity is further coupled to said port.
 8. The system ofclaim 7, wherein said inlet and port are configured such that said porthas a net fluid flow into said casing.
 9. The system of claim 1, whereinsaid signal processor comprises code for determining a turbinerotational speed responsively to said pressure transducer output. 10.The system of claim 1, wherein said signal processor comprises a memorystoring data indicative to said engine.
 11. The system of claim 1,wherein said pressure transducer has a frequency response of about 5 to10 times that of the blade passing frequency.
 12. A method fordetermining turbine blade clearance in a turbomachine comprising aplurality of radially extending blades within a stationary turbinecasing, said method comprising; mounting at least one pressure sensingtransducer on at least one interior wall within a port of said turbinecasing, said pressure transducer mounted substantially adjacent to saidcasing; sensing, by said at least one pressure sensing transducer, afirst pressure signal caused by an unsteady pressure field on the casingas a function of tip clearance size as a corresponding blade passes saidtransducer and a second pressure signal caused by a steady pressurefield on the casing; and determining a clearance between said casing andsaid plurality of blades dependent upon said first and second pressuresignals, wherein said pressure transducer has a frequency responsecapability which is a multiple of the blade passing frequency.
 13. Themethod of claim 12, wherein said pressure transducer has a frequencyresponse of about 5 to 10 times that of the blade passing frequency. 14.The method of claim 12, further comprising the step of providing a firstair chamber substantially adjacent en exterior wall of said port, saidair chamber having an inlet port for receiving cooling air forcirculation within said chamber, and an outlet port for allowing saidcirculated air to exit said chamber.
 15. The method of claim 14, whereinsaid cooling air is around 1300° F.
 16. The method of claim 14, whereinsaid outlet is external to said casing.
 17. The method of claim 14,wherein said inlet and said port are configured such that said port hasa net air flow into said casing.
 18. The method of claim 12, furthercomprising adjusting the blade tip clearance in response to saidgenerated signal.
 19. The method of claim 12, wherein said turbomachinefurther comprises a compressor, and said inlet is substantially adjacentto said compressor.
 20. The method of claim 12, further comprisingcoupling a signal processor to said pressure sensing transducer.
 21. Themethod of claim 20, wherein said signal processor comprises code fordetermining a turbine rotational speed responsively to said output. 22.The method of claim 21, wherein said signal processor comprises a memorystoring data indicative of said turbomachine.
 23. An engine systemcomprising: a compressor, a combustion area and turbine, said turbinehaving a plurality of blades turning inside a casing; a port in saidturbine casing; and a pressure sensor coupled substantially adjacent tosaid casing and including at least one output; said pressure sensormounted in said port wherein a first pressure indicative of a clearancebetween said turbine blades and said casing and a second pressureindicative of an ambient pressure is communicated to said pressuresensor through said port; wherein said pressure sensor has a frequencyresponse capability which is a multiple of the blade passing frequency;and a processor for receiving said first and second pressures anddetermining a clearance between said blades and cases based on saidreceived pressures.
 24. The engine of claim 23, wherein said pressuretransducer has a frequency response of about 5 to 10 times that of theblade passing frequency.
 25. The engine of claim 24, further comprising:a cooling cavity substantially surrounding said pressure sensor, and aninlet for receiving compressed fluid from said compressor and feedingsaid compressed fluid to said cooling cavity to cool said pressuresensor.
 26. The engine of claim 25, wherein said compressed fluidcomprises air.